Recently, a small-size, inexpensive L C noise filter has been proposed which is not liable to produce ringing or the like and yet can positively eliminate penetrating noises.
This noise filter is of a folded type and comprises a first insulating sheet having a first conductor of a rectangular toothed edge-like configuration mounted on one side thereof, and a second insulating sheet having a second conductor of a rectangular toothed edge-like configuration mounted on one side thereof, the two insulating sheets being folded and laminated, with the two conductors staggered relative to each other. In the L C noise filter, constructed as above described, the first conductor functions as a coil having a predetermined number of turns, and the second conductor forms a capacitance relative to the first conductor.
For the purpose of fabricating such a noise filter, it may be conceivable to employ, as an apparatus for folding an insulating sheet in a zigzag fashion, one including a pair of patterns having wave-like alternate ridges and furrows formed on their surface which are adapted to enclose the insulating sheet therebetween.
However, when an insulating sheet is placed between a pair of patterns having alternate ridges and furrows formed thereon, portions at the folds of the sheet become fixed earlier than other portions in flat state. Therefore, if the patterns are moved toward each other when the sheet is in such condition, planar portions between adjacent folds become stretched.
In contrast, if the pair of patterns formed with alternate ridges and furrows is moved to hold therebetween the insulating sheet while the sheet remains unstrained, it is difficult to cause respective ridges and furrows of the patterns to be brought into complete contact because of dimensional tolerances of the pattern pair. Therefore, it is impracticable to give precise folds to the insulating sheet.